Multizone downcomer for slurry hydrocarbon syntheses process

ABSTRACT

A downcomer for producing at least two slurries having different solids and gas concentrations from a single three phase slurry of particulate solids and gas bubbles in a slurry liquid has two or more concentric gas and solids disengaging zones, each having an open fluid conduit depending from an orifice in the bottom. The dowwncomer is useful in a slurry hydrocarbon synthesis process for forming a catalyst and gas reduced slurry which is passed to a liquid filter to remove hydrocarbon liquid from the slurry reactor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a multizone downcomer for a three phase liquidslurry. More particularly the invention relates to a downcomer forproducing at least two different solids and gas reduced slurries from ahydrocarbon synthesis slurry comprising gas bubbles and particulatesolids dispersed in a hydrocarbon liquid.

2. Background of the Invention

Slurry hydrocarbon synthesis (HCS) processes are known. In a slurry HCSprocess a synthesis gas (syngas) comprising a mixture of H₂ and CO isbubbled up as a third phase through a slurry in a reactor in which theslurry liquid comprises hydrocarbon products of the synthesis reactionand the dispersed, suspended solids comprise a suitable Fischer-Tropschtype hydrocarbon synthesis catalyst. Reactors which contain such a threephase slurry are sometimes referred to as "bubble columns", as isdisclosed in U.S. Pat. No. 5,348,982. Irrespective of whether the slurryreactor is operated as a dispersed or slumped bed, the mixing conditionsin the slurry will typically be somewhere between the two theoreticalconditions of plug flow and back mixed. The catalyst particles aretypically kept dispersed and suspended in the liquid by the liftingaction of the syngas bubbling up through the slurry and by hydraulicmeans. Mechanical means such as impellers and propellers and the likeare not used, because they will quickly erode and also cause attritionof the catalyst particles. One or more vertical, gas disengagingdowncomers may be used as hydraulic means to assist in maintaining moreuniform catalyst dispersion, by providing a vertical catalystcirculation in the slurry, as is disclosed in U.S. Pat. No. 5,382,748.The larger catalyst particles tend be more concentrated at the bottom ofthe slurry. It would therefore be advantageous in maintaining a moreuniform vertical catalyst concentration to circulate a catalyst leanslurry to the bottom of the reactor and a catalyst rich slurry at thetop. Further, the slurry liquid hydrocarbon product of the HCS reactionmust be separated from the catalyst particles. This is typicallyaccomplished by mechanical filtration in which the slurry is fed to oneor more porous filter media which permit the liquid to pass through, butnot the catalyst particles. The hydrocarbon liquid filtrate is then sentto further processing and upgrading. The build-up of a catalyst particlecake and plugging of the filters could be reduced if some of thecatalyst particles were removed from the slurry before it is filtered.Hence, there is a need for a means of reducing the catalystconcentration in slurry being fed to the bottom of the reactor and tofiltration.

SUMMARY OF THE INVENTION

The present invention relates to a means and a process forsimultaneously producing slurries of two or more different compositionsfrom a three phase slurry and is useful in a hydrocarbon synthesis (HCS)process. The slurry comprises gas bubbles and particulate solids in aslurry liquid. The means comprises two or more successive gas and solidsdisengaging zones, and at least one slurry transfer conduit. The processcomprises passing the slurry successively through at least two gas andsolids disengaging zones to form a solids and gas reduced slurry, whichis then passed to a desired location by means of a fluid conduit, suchas a downcomer. This invention is useful for removing gas bubbles andsolid catalyst particles from an HCS slurry being fed to the bottom ofthe slurry body to improve vertical catalyst distribution, to slurryfiltration for recovering the slurry liquid produced by the HCS reactionand also for feeding a gas reduced and catalyst increased slurry torejuvenation. By successive zones is meant that slurry passes from theouter zone to an adjacent downstream zone and then to successivelyadjacent downstream zones, until it finally passes into the inner orlast zone, which is furthest downstream. Adjacent zones are in fluidcommunication with each other, by which is meant that slurry passes fromeach zone into the next successive zone downstream. Gas reduced slurriesmay be produced which have a solids concentration both greater and lessthan that in the slurry body which feeds the means. As slurrysuccessively passes from one zone to the next, adjacent downstream zone,the gas concentration in the slurry is reduced. Each zone also hasdowncomer means for feeding slurry from that zone to a desired locationsuch as the top or bottom of the slurry, to filtration, to a zone orvessel external of the slurry, etc. Depending on the configuration ofthe zone and the means that separates it from an adjacent downstreamzone, the solids concentration of the slurry fed into its downcomer maybe greater than that in the slurry fed into the adjacent downstreamzone. The gas and solids disengaging zones may simply be upward opencups having a respective one or more downcomers depending from thebottom, in which gas and solids are disengaged from the slurry passingthrough. In one embodiment, the process of the invention is achieved bymeans which comprises a downcomer immersed in a slurry body, with thetop of the downcomer opening into a multizone gas and solids disengagingmeans comprising an inner zone peripherally surrounded by an outer zoneand separated from the outer zone by weir means. Slurry from the slurrybody surrounding the downcomer passes into the outer zone in which aportion of the gas bubbles and solids are disengaged to form twoslurries which are; (i) a solids and gas reduced slurry which passesover the weir means into the inner zone, and (ii) a gas reduced andsolids increased slurry which is passed out the bottom of the outer zoneto a desired location. In the inner zone, additional gas is disengagedfrom the solids and gas reduced slurry which enters it from the outerzone, to form a gas and solids depleted slurry which is passed to andthrough one or more inner zone downcomer conduits to the desiredlocation.

With particular regard to a slurry HCS process for forming hydrocarbons,at least a portion of which are liquid, the invention comprises thesteps of:

(a) contacting a synthesis gas (syngas) comprising a mixture of H₂ andCO with a solid, particulate hydrocarbon synthesis catalyst in a slurrybody comprising the catalyst and gas bubbles in a hydrocarbon slurryliquid under reaction conditions effective to form hydrocarbons from thesyngas, at least a portion of which are liquid at the reactionconditions and comprise the slurry liquid;

(b) passing a portion of slurry from the slurry body into a first zoneto disengage and remove a portion of the gas bubbles and form a firstgas and catalyst reduced slurry;

(c) passing said first gas and solids reduced slurry into at least oneadjacent zone downstream of the first zone to disengage and remove moregas bubbles in each zone and form successive gas reduced slurries, and

(d) passing slurry from at least one zone into a fluid conduit by whichit is passed to a desired location.

In one embodiment, solids are removed from the slurry in at least onezone, so that the slurry in the innermost or downstream zone is reducedin both gas and solids. In the embodiment referred to above in which atwo zone, gas and solids disengaging means is used, a portion of theslurry body is passed into the outer zone in which a portion of the gasbubbles and solids are disengages from the slurry to form a solids andgas reduced slurry which is passed over a weir means to the inner zone,in which more gas is disengaged from the slurry to form a solids and gasreduced slurry. The solids and gas reduced slurry is then passed intoand through at least one downcomer depending from at least one orificein the bottom of the inner zone, and from there to a desired location,such as to the bottom of the slurry or to filtration means. The slurryremoved from the bottom of the outer zone has less gas bubbles and morecatalyst than the surrounding slurry body, and more gas bubbles andcatalyst than the slurry passing out of the inner zone. In oneembodiment, the outer zone and one or more bottom orifices are sized toincrease the catalyst concentration in the slurry passing out of thatzone into one or more downcomers. This slurry gas reduced and catalystincreased slurry may be passed to a position near the top of the slurry,where catalyst concentration is less than at the bottom, or it may besent to means for rejuvenating the catalyst in the slurry. The slurry orslurry body from which a portion is passed into the solids and gasdisengaging zones may be the reactive slurry in the HCS reactor or itmay be a slurry body in an external rejuvenation or filtration zone andthe fluid conduit may be a simple downcomer. Reducing the solids contentof slurry sent to filtration reduces the buildup of catalyst cake on thefilter and results in greater liquid throughput. During operation of theslurry solids and gas reducing downcomer, the HCS reactor may beoperating or it may be shut down. If it is operating, the presence ofthe downcomer in the reactive HCS slurry does not disturb the HCSreaction. When the process and means of the invention is used to improvethe vertical catalyst concentration in a disperse slurry reactor, thevertical temperature profile in the reactor is more uniform. Thisreduces hot spots and their concomitant lower selectivity to the moredesired liquid hydrocarbon products. The reduction of hot spots alsoreduces catalyst deactivation. Catalyst deactivation increasesselectivity to lower molecular weight products due to lower COconversion. Consequently, hot spot reduction helps to maintain high COconversion and selectivity to higher molecular weight products. A highcatalyst concentration proximate the bottom of the reactor makes it moredifficult to remove the exothermic heat of the HCS reaction, since thespace available for heat exchangers is severely limited. This can resultin the entire lower portion of the reactor running too hot, or the restof the reactor running too cool to avoid heat build-up at the bottom.The invention reduces this by sending a catalyst reduced slurry to thebottom. While the process and means of the invention are describedherein with particular reference to their usefulness in association witha slurry HCS process, the invention is not intended to be so limited.Thus, the invention may be practiced with other types of slurries andchemical processes, including biological and waste water treatmentprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a schematic cross-section of a slurrytype HCS reactor containing a downcomer useful in the practice of theinvention.

FIGS. 2(a) and 2(b) respectively show a schematic cross-section and topview of the downcomer of FIG. 1 in greater detail.

FIGS. 3(a) and 3(b) respectively illustrate a schematic cross-sectionand top view of an embodiment of a disengaging means of the inventioncontaining a slurry turbulence reducing means in the disengaging cup.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates, in cross-section, a slurry type HCSreactor 50 comprising an outer shell 52 containing a three phase HCSslurry 54 and a downcomer 10 useful in the practice of the inventionwithin. The slurry comprises a hydrocarbon slurry liquid in which isdispersed and suspended a particulate HCS catalyst and gas bubbles, thegas bubbles comprising syngas and HCS product gas. The slurry liquidcomprises HCS reaction products which are liquid at the reactionconditions. A syngas feed inlet 56 at the bottom passes the gas up intothe slurry via suitable gas distribution means arrayed across anotherwise gas and slurry impervious tray 58 located at the bottom of theslurry in the reactor. The syngas feed, which comprises a mixture of H₂and CO, bubbles up through the slurry contacting the particulatecatalyst to form hydrocarbon liquids, some hydrocarbon gas and watervapor. HCS reaction products which are gaseous at the reactor conditionsand unused HCS syngas rise up and disengage from the top 60 of theslurry, pass up into gas disengaging and collecting zone 62, and areremoved from the reactor via line 64. Not shown is filtration means,such as one or more liquid filters in the reactive slurry or in one ormore filtration vessels external of the reactor. Such filtration meansseparate the hydrocarbon slurry liquid from the catalyst particles asfiltrate, and pass the filtrate to further processing and upgrading.Magnetic means may also be used to separate the catalyst particles fromthe hydrocarbon liquid product if the catalyst particles are magnetic orparamagnetic, as is disclosed in the prior art. A downcomer 10 of theinvention is shown as totally immersed in the slurry and comprises a gasand solids disengaging means 12 which, in this embodiment and as furtherillustrated in more detail in FIGS. 2(a) and 2(b), is a generallyrectangular shaped cup. One or more downcomers may be employed, only oneof which is shown for the sake of convenience. The interior of the gasand solids disengaging cup 12 comprises outer and inner gas and solidsdisengaging zones zone 18 and 20, respectively, defined by outer andinner walls 22 and 24 and inner and outer zone bottoms 26 and 28. Theinner zone is peripherally surrounded by the outer zone as shown in FIG.2(b). Zones 18 and 20 open at the bottom through respective orifices 17,19 and 21 into respective hollow downcomer conduits or tubes 13, 14, 15,16, and 17 which depend vertically down from the bottom of the zones.Only 14, 15 and 16 are shown in FIGS. 1, 2(a) and 3(a) for the sake ofconvenience. All four are shown in FIG. 2(b). Inner wall 24 alsofunctions as a weir for gas and solids reduced slurry present in outerzone 18 to flow over and into inner zone 20, as generally illustrated byarrows 40 in FIG. 2(a). Downcomer conduits 14, 15 and 16 permit slurryin the respective outer and inner zones to flow down out of the zones toa desired location. The top of outer and inner walls 22 and 24 may beserrated or castellated to permit a smoother flow of slurry over thetops thereof. In the embodiment illustrated in FIG. 1, the slurrypassing down through downcomers 14 and 15 from outer zone 18 contains agreater concentration of catalyst than both the slurry passing downthrough the inner zone downcomer 16 and the surrounding slurry 54. Inthis embodiment the length of downcomers 14 and 15 is substantiallyshorter than the length of downcomer 16 which extends almost to thebottom of slurry body 54, so that the slurry containing the higherconcentration of catalyst is passed into the upper portion of the slurrywhere the catalyst concentration is normally least and the slurrycontaining the lower catalyst concentration is passed out the bottom ofdowncomer 16 proximate the bottom of the slurry where the catalystconcentration is normally the greatest, to reduce catalystmaldistribution in the slurry. A baffle in the form of, for example, asimple plate or cone 30 below the bottom opening 32 of downcomer 16,prevents feed gas from entering the downcomer without inhibiting slurryflow out of the downcomer and into the bottom of the slurry in thereactor. If syngas enters up into the downcomer it can not only inhibitand even prevent slurry flow down and out the downcomer, it can alsoforce the slurry back up the downcomer, due to the lifting action of thegas. An inverted funnel or generally cone shaped gas and slurry baffle34 terminating upward in gas disengaging zone 62 via a hollow chimney orgas conduit 36 is positioned over the gas disengaging means 12, with itsbottom opening 38 below the top 60 of slurry body 54, to provide aslurry flow path 38 from the slurry body 54 into the outer zone 18 and agas collecting and removal zone 33 above zones 18 and 20. FIG. 3(a)shows a further embodiment of downcomer 10 of the invention in which aslurry turbulence eliminating means 60 is positioned inside the slurrydisengaging cup proximate the top thereof, for minimizing the effects ofsurges, splashes and other slurry turbulence in the main slurry bed thatmay find their way into disengaging means 12 and otherwise disturb thegas and solids release in the disengaging zones. Thus, turbulencereducer 60 aids in maintaining quiescent zones inside the disengagingcup which provides more efficient gas and catalyst disengagement. Asshown in FIGS. 3(a) and 3(b), the turbulence reducing means may comprisea grid comprising a plurality of strips 62 arrayed at right angles toeach other and mutually intersecting each other (not shown) in a mannersimilar to that of separators in a beverage carton and/or weldedtogether, to form a plurality of open cells 66, of which only a few arelabeled for the sake of convenience. These cells permit the flow ofslurry down into the zones and at the same time minimize flowperturbations from disrupting the quiescence of the slurry in the zones.In another embodiment (not shown) of a downcomer of the inventionsimilar in almost all respects to that of FIG. 1-3, the top of innerwall 24 is higher than the top of the outer wall 22 and one or moreapertures or slots circumferentially present in the inner wall 24 permitslurry to flow from the outer zone 18 into the inner zone 20. Theseslots or apertures are located at a point below the top of the innerwall. In this embodiment, slurry cannot pass over the top of the innerwall into the inner zone. Instead, the slurry from the outer zone passesinto the inner zone through the plurality of orifices or slots presentin the inner wall. In this embodiment, the high inner wall acts as abarrier to slurry surges, etc. from the main slurry body in the reactor,from entering the inner zone, thereby further minimizing flow turbulenceand perturbations in the inner zone which can be caused by slurrypassing over the top of the inner wall and down into the slurry below inthe inner zone.

Slurry entering the gas and solids disengaging means of the inventionwill begin to release gas bubbles as soon as it is out of contact withthe gas bubbles rising up through the slurry and out of the reactor.However, surges, splashes and other slurry flow turbulence in the slurrybody surrounding the gas and solids disengaging means will, if permittedto do so, cause gas and solids laden slurry from the main body to mixwith the slurry in the otherwise quiescent disengaging zones. If thishappens, the fresh slurry with its gas bubbles and catalyst solids willsimply replace some or all of the slurry in the disengaging zones andreduce the effectiveness of the processes and disengaging means of theinvention. The uprising gas bubbles in the slurry also serve to maintainthe catalyst particles dispersed in the reactor slurry. However, ofitself this isn't completely effective, as the vertical slurryconcentration gradient known as slurry maldistribution may occur despitethe uprising gas bubbles. Hence, slurry downcomers are used to reducethe slurry maldistribution as disclosed in U.S. Pat. No. 5,382,748. Inoperation, slurry flows into the outer disengaging zone (or, in the caseof more than two disengaging zones, the first such zone) where degassingand letdown of the catalyst particles which are heavier than the liquidbegins to take place. As this occurs, a gas reduced and catalystenriched slurry is formed near the bottom of the outermost zone. Thisslurry exits the outermost zone via its one or more associateddowncomers or other slurry transfer means at the bottom of the zonewhich, in the embodiments shown, pass the catalyst enriched slurry onlya short distance below the top of the slurry body surrounding the means,which is where the catalyst concentration is normally least in adisperse slurry bed, rather than proximate the bottom of the reactorwhere the catalyst concentration is normally greatest. If desired, allor a portion of this catalyst enriched slurry in which the catalystconcentration is greater than that in the surrounding slurry body can bepassed into a suitable slurry catalyst rejuvenation zone to at leastpartially rejuvenate the catalyst particles in the slurry. In theembodiment of FIG. 2, gas and catalyst reduced slurry in the upperportion of the outer or first disengaging zone continuously flows overthe outer wall, which is a weir, and into the inner or seconddisengaging zone, as fresh slurry from the main slurry body continues toflow over the top of the outer wall and into the outer zone. In theinner or second zone, more gas bubbles are released from the slurry toform a gas and catalyst reduced slurry. This gas and catalyst reducedslurry passes down into the inner zone downcomer conduit and exitsproximate the bottom of the slurry in the reactor where the catalystconcentration is normally least. This slurry circulation through thedowncomer assists in achieving a more uniform vertical distribution ofthe catalyst in the slurry body.

In determining the actual sizing, shape and design of the gas and solidsdisengaging means of the invention, a number of factors are taken intoaccount. The outer zone is sized to allow sufficient residence time topermit catalyst settling to the extent necessary for the catalystreduced slurry to pass over the weir and into the inner zone, while thecombined sizes of the outer and inner zones must provide sufficientslurry residence time/downward flow combination, to permit the slurry todegas to the desired extent. The cap illustrated in FIG. 2 is wideenough to cover the outer perimeter of the outer zone, with the chimneysized to allow removal of the escaping gas released from the slurry inthe disengaging zones. The annular space between the inner surface ofthe cap and the outer perimeter of the disengaging means, or the outerperimeter of the first or outer zone, must be large enough to permitsufficient slurry flow into the disengaging means to satisfy the otherrequirements. Studies were made for a 3 inch downcomer of the prior arthaving a 2 foot diameter gas disengaging cup at the top. Immersed in anHCS slurry comprising 60 volume % gas bubbles the slurry flowing downthrough the downcomer would have a gas bubble concentration of only20-40% and the hydraulic velocity of the slurry flowing down through thedowncomer would be from 8-16 feet/second.

In an HCS process, liquid and gaseous hydrocarbon products are formed bycontacting a syngas comprising a mixture of H₂ and CO with a suitableFischer-Tropsch type HCS catalyst, under shifting or non-shiftingconditions and preferably under non-shifting conditions in which littleor no water gas shift reaction occurs, particularly when the catalyticmetal comprises Co, Ru or mixture thereof. Suitable Fischer-Tropschreaction types of catalyst comprise, for example, one or more Group VIIIcatalytic metals such as Fe, Ni, Co, Ru and Re. In one embodiment thecatalyst comprises catalytically effective amounts of Co and one or moreof Re, Ru, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganicsupport material, preferably one which comprises one or more refractorymetal oxides. Preferred supports for Co containing catalysts comprisetitania, particularly when employing a slurry HCS process in whichhigher molecular weight, primarily paraffinic liquid hydrocarbonproducts are desired. Useful catalysts and their preparation are knownand illustrative, but nonlmiting examples may be found, for example, inU.S. Pat. Nos. 4,568,663; 4,663,305; 4,542,122; 4,621,072 and 5,545,674.

The hydrocarbons produced by an HCS process according to the inventionare typically upgraded to more valuable products, by subjecting all or aportion of the C₅₊ hydrocarbons to fractionation and/or conversion. Byconversion is meant one or more operations in which the molecularstructure of at least a portion of the hydrocarbon is changed andincludes both noncatalytic processing (e.g., steam cracking), andcatalytic processing (e.g., catalytic cracking) in which a fraction iscontacted with a suitable catalyst. If hydrogen is present as areactant, such process steps are typically referred to ashydroconversion and include, for example, hydroisomerization,hydrocracking, hydrodewaxing, hydrorefining and the more severehydrorefining referred to as hydrotreating, all conducted at conditionswell known in the literature for hydroconversion of hydrocarbon feeds,including hydrocarbon feeds rich in paraffins. Illustrative, butnonliniting examples of more valuable products formed by conversioninclude one or more of a synthetic crude oil, liquid fuel, olefins,solvents, lubricating, industrial or medicinal oil, waxy hydrocarbons,nitrogen and oxygen containing compounds, and the like. Liquid fuelincludes one or more of motor gasoline, diesel fuel, jet fuel, andkerosene, while lubricating oil includes, for example, automotive, jet,turbine and metal working oils. Industrial oil includes well drillingfluids, agricultural oils, heat transfer fluids and the like.

It is understood that various other embodiments and modifications in thepractice of the invention will be apparent to, and can be readily madeby, those skilled in the art without departing from the scope and spiritof the invention described above. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the exactdescription set forth above, but rather that the claims be construed asencompassing all of the features of patentable novelty which reside inthe present invention, including all the features and embodiments whichwould be treated as equivalents thereof by those skilled in the art towhich the invention pertains.

What is claimed is:
 1. A slury hydrocarbon synthesis proccss for forminghydrocarbons comprising:(a) contacting a synthesis gas comprising amixture of H₂ and CO with a solid, particulate hydrocarbon synthesiscatalyst in a slurry body which comprises said catalyst and gas bubblesin a slurry liquid under reaction conditions effective to from saidhydrocarbons from said synthesis gas, at least a portion of which areliquid at said reaction conditions and comprise said slurry liquid; (b)passing a portion of slurry from said slurry body into a first zone todisengage and remove a portion of said gas bubbles and catalyst to form(i) a gas and catalyst reduced slurry from one portion of slurry in saidzone and (ii) a gas reduced and catalyst increased slurry from anotherportion and returning said gas reduced and catalyst increased slurryback into said slurry body; (c) passing said first gas and solidsreduced slurry into at least one adjacent zone downstream of said firstzone thereby creating at least two adjacent gas disengaging zones todisengage and remove more gas bubbles in each said zone and formsuccessive gas reduced slurries, and (d) passing slurry from at leastone said zone into a fluid conduit by which it is passed in a desiredlocation.
 2. A process according to claim 1 wherein said catalyst andgas reduced slurry is passed to the lower portion of said slurry body.3. A process according to claim 1 wherein said catalyst and gas reducedslurry is passed to slurry liquid filtration means.
 4. A processaccording to claim 3 wherein said slurry is passed through more than twosaid zones.
 5. A process according to claim 1 wherein said first zonealso produces a slurry which has a catalyst concentration greater thanthat in said slurry body.
 6. A process according to claim 1 wherein saidcatalyst comprises at least one supported Group VIII metal.
 7. A processaccording to claim 2 wherein at least a portion of said hydrocarbonsformed from said synthesis gas are upgraded by one or more conversionoperations to at least one more valuable product.